U.S. patent application number 13/300056 was filed with the patent office on 2012-05-24 for computer-implemented method and system for automatic generation of time slot allocation for a wireless control loop.
Invention is credited to Alf Isaksson.
Application Number | 20120127971 13/300056 |
Document ID | / |
Family ID | 41381654 |
Filed Date | 2012-05-24 |
United States Patent
Application |
20120127971 |
Kind Code |
A1 |
Isaksson; Alf |
May 24, 2012 |
Computer-Implemented Method And System For Automatic Generation Of
Time Slot Allocation For A Wireless Control Loop
Abstract
A computer implemented method for providing a time slot
allocation in a wireless communication schedule for monitoring and
control of a control loop in an industrial process, the industrial
process having at least one control task and a plurality of
wirelessly enabled field devices. The method includes importing
dependency information between at least two field devices of at
least one control loop, automatically converting the dependency
information into a communication schedule and allocating time slots
in the communication schedule to one or more field devices of the
at least one control loop dependent at least on a dependency
relationship between at least two field devices. A system and a
computer program product are also disclosed.
Inventors: |
Isaksson; Alf; (Vasteras,
SE) |
Family ID: |
41381654 |
Appl. No.: |
13/300056 |
Filed: |
November 18, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/EP2009/056065 |
May 19, 2009 |
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13300056 |
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Current U.S.
Class: |
370/337 ;
370/336 |
Current CPC
Class: |
H04W 72/12 20130101;
H04W 72/0446 20130101 |
Class at
Publication: |
370/337 ;
370/336 |
International
Class: |
H04J 3/00 20060101
H04J003/00 |
Claims
1. A computer implemented method for providing a time slot
allocation in a wireless communication schedule for monitoring and
control of a control loop in an industrial process, said industrial
process having at least one control task and a plurality of
wirelessly enabled field devices characterized by importing
dependency information between at least two said field devices of
at least one said control loop from any of a CAD file, a P&I
diagram in electronic file form, a process logic diagram in
electronic file form, a P&I diagram in the form of an extended
XML file, a process graphic, a controller configuration in a
control system, and allocating time slots in said communication
schedule to one or more said field devices of the at least one said
control loop dependent at least on a dependency information between
at least two said field devices.
2. The method according to claim 1, characterised by allocating
time slots in a said wireless communication schedule arranged to
one or more said field devices in two or more said control
loops.
3. The method according to claim 1, characterised by allocating
time slots in a said wireless communication schedule arranged as a
superframe to one or more said field devices in at least one said
control loop where the superframe comprises both a said wireless
communication schedule and a control task execution schedule of a
controller.
4. The method according to claim 1, characterised by allocating
time slots in two or more said wireless communication schedules
arranged as multiple superframes to one or more said field devices
in two or more said control loops.
5. The method according to claim 1, characterised by sending from a
wireless gateway in a wireless network a time stamp or clock signal
to a controller of a said control loop and synchronizing the
execution of a control cycles of the controller to the time slots
of said wireless communication.
6. The method according to claim 1, characterised by allocating
time slots in one or more TDMA schemas for a spread-spectrum
wireless network operating as a mesh network and using frequency
hopping to allocate channels.
7. The method according to claim 1, characterised by allocating
time slots in one or more TDMA schemas for a wireless network
compatible with WirelessHART.
8. The method according to claim 1, characterised by allocating
time slots in one or more TDMA schemas for a wireless network
compatible with an ISA 100 standard.
9. The method according to claim 1, characterised by converting the
dependency information between said field devices of at least one
said control loop into one or more dependency relationships.
10. The method according to claim 1, characterised by converting
dependency information between said field devices of at least one
said control loop, the dependency information being comprised in a
process logic format, into one or more dependency relationships and
allocating time slots in said communication schedule to one or more
said field devices dependent at least on the dependency
relationship between at least two said field devices.
11. The method according to claim 1, characterised by allocating
time slots for field devices in the one or more control loops in
one of a plurality of superframes, which allocation is dependent at
least on the information of dependency between two or more field
devices and/or two or more control loops.
12. The method according to claim 1, characterised by storing one
or more TDMA schemas or superframes in a wireless gateway or
wireless node configured securely connected to a Network Manager
application or device.
13. The method according to claim 1, characterised by allocating at
least one shared time slot for a field device in the one or more
control loops in at least one of a plurality of superframes.
14. A wireless communication system for monitoring and control of a
control loop of an industrial process, said industrial process
having at least one control task and a plurality of wireless
enabled field devices, said wireless communication system
comprising a plurality of wireless nodes communicating according to
a wireless communication schedule and forming part of a wireless
network, characterised in that the wireless network is controlled
by a wireless network manager dependent on time slots allocated in
said communication schedule to one or more said field devices of at
least one said control loop dependent at least on a dependency
information between at least two said field devices, and a
distributed control system having an application or function into
which dependency information between at least two said field
devices of at least one said control loop is arranged to be
imported from any of a CAD file, a P&I diagram in electronic
file form, a process logic diagram in electronic file form, a
P&I diagram in the form of an extended XML file, a process
graphic, a controller configuration in a control system.
15. The system according to claim 14, characterised in that the
wireless network manager controls the wireless network dependent on
time slots allocated in said wireless communication schedule to one
or more said field devices of at least one said control loop
dependent at least on a dependency information between at least two
said field devices and two or more control loops.
16. The system according to claim 14, characterised in that the
wireless network manager controls the wireless network dependent on
time slots allocated in said wireless communication schedule to one
or more said field devices of at least one said control loop
dependent at least on a dependency information between two or more
control loops.
17. The system according to claim 14, characterised in that said
wireless communication schedule is arranged as a superframe to one
or more said field devices in at least one said control loop where
the superframe comprises both a said wireless communication
schedule and a control task execution schedule of a controller.
18. The system according to claim 14, characterised in that at
least one wireless gateway in the wireless network is arranged
configurable for clock synchronization with a control task of the
one or more said control loops.
19. The system according to claim 18, characterised in that the
network manager is arranged connected to the at least one wireless
gateway in the wireless network using a secure connection.
20. The system according to claim 14, characterised in that the
network manager is arranged incorporated in a wireless gateway in
the wireless network.
21. The system according to claim 14, characterised in that a
handheld wireless configuration device is arranged to configure any
of a network manager, gateway or wirelessly enabled field
device.
22. The system according to claim 14, characterised by one or more
field devices connected to said control loop of an industrial
process by a hard-wired connection.
23. The system according to claim 14, characterised by a processing
unit in a computer based system, the processing unit having an
internal memory with a computer program product loaded therein,
comprising software code portions for allocating time slots in said
communication schedule to one or more said field devices of the at
least one said control loop dependent at least on a dependency
information between at least two said field devices stored on a
memory storage device connected to the control system or DCS.
24. A computer program product on a data carrier comprising
computer program code configured to perform a method for providing
a time slot allocation in a wireless communication schedule for
monitoring and control of a control loop in an industrial process,
said industrial process having at least one control task and a
plurality of wirelessly enabled field devices, characterized by
importing dependency information between at least two said field
devices of at least one said control loop from any of a CAD file a
P&I diagram in electronic file form, a process logic diagram in
electronic file form, a P&I diagram in the form of an extended
XML file, a process graphic, a controller configuration in a
control system, and allocating time slots in said communication
schedule to one or more said field devices of the at least one said
control loop dependent at least on a dependency information between
at least two said field devices, when said program code is loaded
into a computer or terminal connected to a process control system.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of pending
International patent application PCT/EP2009/056065 filed on May 19,
2011 which designates the United States and the content of which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is concerned with wireless
communication in an industrial automation context. In particular it
is concerned with a method and system for automatic generation of
time slot allocation in one or more wirelessly enabled control
loops which are monitored and/or controlled by a process control
system.
BACKGROUND OF THE INVENTION
[0003] Process control for industrial automation processes or
industrial automation devices is often supervised and regulated by
a process control system as a number of control loops including one
or more closed loop control processes. A traditional approach in
the use of closed loop control is to measure a value of a process
output and compare the measured value with a reference value. There
are also other objectives of control loop control, including
set-point regulation, tracking (time-varying reference path), path
following (varying reference independent of time), disturbance
attenuation etc. The most common form of control is a Proportional,
Integral, Derivative (PID) control for feedback control. In PID
control a sensor measurement is used as an input for a feedback
control loop, and any difference between the measured sensor value
and a reference (setpoint) value or signal is determined by a
controller. The controller then in turn sends signals to an
actuator connected to the control loop in question, making changes
to the process, so that the sensed value approaches the reference
value over time.
[0004] The traditional closed loop feedback system comprises
hard-wired communication links. A disadvantage with hard-wired
communication links is that changes in position of any component or
field device in the closed loop, such as a sensor or actuator,
usually requires a stop in production or an extensive shutdown,
especially in the case of analogue wired connections, and/or
digital wired connections. Alternatively, such changes have to be
delayed until a process shutdown may be programmed. In addition,
hard wiring may be both expensive to replace and sometimes also
technically challenging to replace.
[0005] Wireless technologies give several advantages to industrial
automation in terms of gain in productivity and flexibility.
Industrial sites are often harsh environments with stringent
requirements on the type and quality of cabling. Moreover large
sites often require many thousands of cables and it could be
difficult to install or engineer additional wires in an already
congested site. Thus wireless communication can save costs and time
during an installation phase. At the same time wireless
communication can improve reliability with respect to wired
solutions by offering several mechanisms of diversity, such as
space diversity, frequency diversity and time diversity.
Furthermore the ad-hoc nature of wireless networks allows for easy
setup and re-configuration when the network grows in size. However,
when the field devices such as sensors and/or actuators are part of
a closed-loop control system, an industrial application will
require hard limits on the maximum delay allowed during the
communication, so strict timing requirements have to be applied and
consistently achieved.
[0006] Another requirement is the coexistence of the network with
other equipment and competing wireless systems. The WirelessHART
standard has been developed to fulfill these demands. WirelessHART
is a wireless mesh network communication protocol for process
automation applications, including process measurement, control,
and asset management applications. It is based on the HART
protocol, but it adds wireless capabilities to it enabling users to
gain the benefits of wireless technology while maintaining
compatibility with existing HART devices, tools and commands. A
WirelessHART network may be connected to the plant automation
network through a gateway. The plant automation network could be a
TCP-based network, a remote I/O system, or a bus such as PROFIBUS.
All network devices such as field devices and access points
transmit and receive WirelessHART packets and perform the basic
functions necessary to support network formation and
maintenance.
[0007] Devices can be deployed in a star topology, that is where
all devices are one hop to the gateway, to support a high
performance application, or in a multi-hop mesh topology for a less
demanding application, or any topology in between. These
possibilities give flexibility to WirelessHART technology enabling
various applications (both high and low performance) to operate in
the same network. WirelessHART specifies the use of IEEE STD
802.15.4-2006 compatible transceivers operating in the 2.4 GHz ISM
(Industrial, Scientific, and Medical) radio band. Communications
among network devices are arbitrated using TDMA (Time Division
Multiple Access) that allows scheduling of the communication link
activity.
[0008] WirelessHART uses TDMA and channel hopping to control access
to the network and to coordinate communications between network
devices. The basic unit of measure is a time slot which is a unit
of fixed time duration commonly shared by all network devices in a
network. The duration of a time slot is sufficient to send or
receive one packet per channel and an accompanying acknowledgement,
including guard-band times for network wide-synchronization. The
WirelessHART standard specifies that the duration of the time slot
is 10 ms. The TDMA Data Link Layer establishes links specifying the
time slot and frequency where 1.3. WirelessHART Standard
communication between devices occurs. These links are organized
into superframes that periodically repeat to support cyclic and
acyclic communication traffic.
[0009] WirelessHART standard does not specify a particular
scheduling algorithm to be used for scheduling communication in a
WirelessHART network. However, for all network devices accessed
through a WirelessHART gateway, the user has to configure how often
each measurement value is to be communicated to the gateway. In
order to support multiple superframes for the transfer of process
measurements at different rates, the size of superframes should
follow a harmonic chain in the sense that all periods should divide
into each other, in particular, scan rates should be configured as
integer multiples of the fastest update time that will be supported
by network devices. The correctness of the process control system
behavior depends not only on the logical results of the
computations performed in each controller, but also on the physical
instant at which these results are produced, in other words, it is
a system with explicit deterministic (or probabilistic) timing
requirements.
[0010] Thus the task of making up a schedule for communication
links in a control loop may be subject to many conditions and
constraints. In addition, in a typical process installation,
communication scheduling and formation of superframes may be
required for many hundreds or even thousands of control loops. This
is a time consuming process for an installer or engineer, and one
that also presents opportunities for errors to be made when an
installer manually constructs a time schedule or configures a
superframe for a control loop based on information for a control
diagram or P&I diagram (Piping and Instrumentation). U.S. Pat.
No. 7,460,865 entitled Self-configuring communication networks for
use with process control systems, assigned to Fisher-Rosemount
Systems, Inc., discloses methods for automatically assigning a
first and a second wireless link to a first field device, wherein
the first wireless link wirelessly couples the first field device
to a second field device and wherein the second wireless link
wirelessly couples the second wireless field device to a
controller; the assignment being dependent on at least one first
predetermined signal criterion.
SUMMARY OF THE INVENTION
[0011] The aim of the present invention is to remedy one or more of
the above mentioned problems. This and other aims are obtained by a
method for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process.
[0012] In a first aspect of the invention a method is disclosed for
providing a time slot allocation in a wireless communication
schedule for monitoring and control of a control loop in an
industrial process, said industrial process having at least one
controller and a plurality of wirelessly enabled field devices
wherein the method comprises importing dependency information
between at least two said field devices of at least one said
control loop, and allocating time slots in said communication
schedule to one or more said field devices of the at least one said
control loop dependent at least on a dependency relationship
between at least two said field devices.
[0013] According to an embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises importing dependency information
between at least two said field devices of at least one said
control loop, and allocating time slots in a said wireless
communication schedule arranged to one or more said field devices
in two or more said control loops.
[0014] According to another embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises allocating time slots in a said
wireless communication schedule arranged as a superframe to one or
more said field devices in at least one said control loop where the
superframe comprises both a said wireless communication schedule
and a control task execution schedule of a controller.
[0015] According to an embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises allocating time slots in two or more
said wireless communication schedules arranged as multiple
superframes to one or more said field devices in two or more said
control loops.
[0016] According to another embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises sending from a wireless gateway in the
wireless network a time stamp or clock signal to a controller of a
said control loop and synchronizing the execution of a control
cycles of the controller to the time slots of said wireless
communication.
[0017] According to an embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises allocating time slots in one or more
TDMA schemas for a spread-spectrum wireless network operating as a
mesh network and using frequency hopping to allocate channels.
[0018] According to another embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises allocating time slots in one or more
TDMA schemas for a wireless network compatible with WirelessHART or
by allocating time slots in one or more TDMA schemas for a wireless
network compatible with an ISA 100 standard.
[0019] According to another embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises converting the dependency information
for said field devices of at least one said control loop into a
graph of dependency relationships.
[0020] In another embodiment of the invention a method is disclosed
for providing a time slot allocation in a wireless communication
schedule for monitoring and control of a control loop in an
industrial process, said industrial process having at least one
controller and a plurality of wirelessly enabled field devices
wherein the method comprises allocating time slots for field
devices in the one or more control loops in one of a plurality of
superframes, which allocation is dependent at least on the
information of dependency between two or more field devices and/or
two or more control loops.
[0021] According to another embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises storing one or more TDMA schemas or
superframes in a wireless gateway or wireless node configured
securely connected to a Network Manager application or device.
[0022] According to another embodiment of the invention a method is
disclosed for providing a time slot allocation in a wireless
communication schedule for monitoring and control of a control loop
in an industrial process, said industrial process having at least
one controller and a plurality of wirelessly enabled field devices
wherein the method comprises importing dependency information on
dependency between each said field device of one or more control
loops from any of: a CAD file, a P&I diagram in any form, a
process logic diagram in electronic file form, a P&I diagram in
the form of an extended XML file, one or more operator process
graphics, a controller configuration in a control system.
[0023] It is advantageous, when scheduling network communication
for wireless control using an industrial wireless standard such as
the recently released WirelessHART standard HART 7 Specifications,
to schedule the sensor and actuator communication for individual
controller tasks together, in one configuration step. To be able to
do this it is necessary to have a description of which sensors and
actuators that belong to the same task. This dependency may be
represented as an acyclic directed graph. In this disclosure it is
described how this dependency graph may be generated automatically
based on either a CAD process and instrumentation diagram (P&I
D) or based on a controller configuration in or created in a
Distributed Control System (DCS).
[0024] For a realistic process control example with hundreds of
control loops it may, however, be both time consuming but more
importantly error prone to schedule the sensor and actuator
communication for individual controller tasks together in a manual
operation. The proposal of this disclosure is to automatically
generate the dependency graph. This may be done either from a
P&I diagram if electronically available, or from a
configuration or a process graphic existing in the DCS.
[0025] A first advantage of creating dependency charts
automatically direct from a P&I diagram or other existing
process control schema is that manual errors are eliminated from
the process. A second advantage is that, although automatic
dependency chart conversion may consume more set-up time, in a more
realistic example where there will be hundreds of control loops
requiring a schedule for the communication, in a superframe, a
considerable amount of time is saved because less time is required
per control loop to automatically convert and create a dependency
chart.
[0026] In another aspect of the present invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is disclosed, said industrial process
having a controller and a plurality of wireless enabled field
devices, said wireless communication system comprising a plurality
of wireless nodes communicating according to a wireless
communication schedule, wherein the wireless network is controlled
by a wireless network manager dependent on time slots allocated in
said communication schedule to one or more said field devices of at
least one said control loop dependent at least on a dependency
relationship between at least two said field devices.
[0027] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein the wireless network is controlled
by a wireless network manager dependent on time slots allocated in
said communication schedule to one or more said field devices of at
least one said control loop dependent at least on a dependency
relationship between at least two said field devices.
[0028] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein the wireless network manager
controls the wireless network dependent on time slots allocated in
said wireless communication schedule to one or more said field
devices of at least one said control loop dependent at least on a
dependency relationship between at least two said field
devices.
[0029] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein said wireless communication
schedule is arranged as a superframe to one or more said field
devices in at least one said control loop where the superframe
comprises both a said wireless communication schedule and a control
task execution schedule of a controller.
[0030] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein at least one wireless gateway in
the wireless network is arranged configurable for clock
synchronization with a controller of the one or more said control
loops.
[0031] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein the network manager is arranged
connected to the at least one wireless gateway (60) in the wireless
network using a secure connection.
[0032] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein the network manager is arranged
incorporated in a wireless gateway in the wireless network.
[0033] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein a handheld wireless configuration
device is arranged to configure any of a network manager, gateway
or wirelessly enabled field device.
[0034] In another embodiment of the invention a wireless
communication system for monitoring and control of a control loop
of an industrial process is described, said industrial process
having at least one controller and a plurality of wireless enabled
field devices, said wireless communication system comprising a
plurality of wireless nodes communicating according to a wireless
communication schedule, wherein one or more computer programs for
carrying out one or more methods of the invention are stored on a
memory storage device connected to the control system or DCS.
[0035] In another aspect of the present invention a portable
apparatus is disclosed for configuring wireless devices for
monitoring and control of a control loop of an industrial
process.
[0036] A computer program, and a computer program recorded on a
computer-readable medium is disclosed in another aspect of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] A more complete understanding of the method and system of
the present invention may be had by reference to the following
detailed description when taken in conjunction with the
accompanying drawings wherein:
[0038] FIG. 1 shows a schematic diagram for a series of control
loops showing in particular a dependency chart of a dependency
relationship between two or more field devices in one or more
control loops according to an embodiment of the invention according
to FIG. 4;
[0039] FIG. 2 shows a schematic diagram of the embodiment of FIG. 1
and in particular for the series of control loops referred to in
FIG. 4 wherein FIG. 2 shows update times or period times for each
of the control loops, in a method according to an embodiment of the
invention;
[0040] FIG. 3 shows a schematic diagram of a development of the
embodiment of FIG. 4 and in particular shows a diagram for a
superframe in which a series of times slots in a TDMA superframe
are allocated to one or more wirelessly enabled field devices in a
control loop in a method according to an embodiment of the
invention;
[0041] FIG. 4 shows a simplified flowchart for a development of one
or more methods according to a preferred embodiment of the
invention in which steps of a method for automatically generating a
wireless communication time slot allocation schema for a control
loop dependent on a dependency between field devices are shown;
[0042] FIG. 5 (Prior Art) shows a schematic diagram of a process
control system in an industrial installation including a series of
control loops comprising one or more wirelessly enabled field
devices;
[0043] FIG. 6 shows a flowchart for another embodiment of the
method of the preferred embodiment of FIG. 4, in which more
particularly optional steps for automatically generating a wireless
communication time slot allocation schema for a control loop
dependent on a dependency between field devices are shown;
[0044] FIG. 7 shows a flowchart for a method according to the
preferred embodiment of FIG. 4, in which steps of a method for
automatically generating a wireless communication time slot
allocation schema for a control loop dependent on a dependency
between control loops are shown in a preferred embodiment of the
invention;
[0045] FIG. 8 is a flowchart for a development of the preferred
embodiment of FIG. 4 or FIG. 7, in which more particularly one or
more control task schedule time slots for controllers in a control
loop dependent on a dependency between control loops are
automatically generated as well as a plurality of wireless
communication time slot allocations;
[0046] FIG. 9 shows a schematic process and instrumentation (P
& I) diagram for an exemplary process comprising a reactor
vessel from which control information may be extracted to describe
one or more process control dependency relationships which may be
automatically converted into a time slot allocation schema using an
embodiment according to FIG. 4 or FIG. 7;
[0047] FIG. 10 is a schematic diagram for the embodiment of FIG. 4
and in particular for an embodiment comprising a display of an
application for configuring a process control system or DCS;
[0048] FIG. 11 shows a second schematic diagram of the embodiment
of FIG. 4 or FIG. 7 which in particular shows dependency
relationships between field devices of the control loops shown in
FIG. 9; and
[0049] FIG. 12 shows a schematic diagram for two superframes in
which a series of timeslots in TDMA superframes are allocated to
one or more wirelessly enabled field devices, as well as the
control tasks, of control loops such as those of FIG. 9 part of a
method according to the preferred embodiment of the invention of
FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
[0050] In this disclosure it is described how a dependency graph
for a control loop is generated automatically. The dependency graph
is based on an existing process description contained, for example,
in a CAD process and instrumentation diagram (P&I D) or based
on a process logic configuration created in a Distributed Control
System (DCS), or another schema detailing the process control logic
for a process. The dependency graph is in turn used to allocate
time slots in a TDMA (Time Division Multiple Access) schema in
order to configure the wireless communication links of the control
loop of a process.
[0051] An example is shown by means of FIGS. 1-3, where controllers
are configured to execute control tasks jointly with wireless
communication schedules for three different scan times in FIG. 3,
corresponding to different superframes in a WirelessHART schedule.
The corresponding dependency graph showing a dependency between one
control loop and another is depicted in FIG. 1 and the superframes
in FIG. 3. For a simple example of this size it is of course
straightforward and fast to simply manually construct and/or enter
the dependencies, and to allocate slots in the superframes. In more
realistic examples containing more complex relationships, and more
control loops, automatically constructing the dependency
relationships and the superframes saves a great deal of time.
[0052] FIG. 1 shows five control loops 21-25. Each control loop
includes wirelessly enabled field devices, for example sensors
S1-S7 and actuators A1-A6. Each control loop includes one or more
control tasks C1-C5 which are typically executed in a Programmable
Logic Controller (PLC) or other industrial controller. Optionally
two or more different control tasks may be carried out by the same
controller. FIG. 1 shows a simple control loop 23 which has one
input from a sensor S4, which is sent to a controller, where the
control carries out a control task C3, and then sends an output
signal to an actuator A4. A control loop may have more than one
input and/or output. Control loop 21 is shown to have two inputs
from two sensors S1 and S2. The control task C1 carried out by the
controller then results in a single output to an actuator A1.
[0053] A dependency exists between some of the control loops in a
typical process in the sense that one control task in a controller
may require input from another control loop in order to complete a
control task. Thus in FIG. 1, control loop 21 requires as an input
a control output from the control task C2 of loop 22, as well as
input from two sensors S1, S2, in order to complete its control
task. The dependency between control loops may also be seen in FIG.
3. FIG. 3 shows a control task execution schedule 36. The schedule
shows that a controller executes its control task C2 (including
input from S3) before the controller executes its control task C1
(including output from the other controller C2).
[0054] FIG. 2 shows the period times or update rates for the five
different control loops 21-25. The fastest control loop 23 has an
update time, scan time, or period of 60 ms; the two slowest loops,
the two loops with the greatest period 240 ms are loops 21, 22. Two
other loops 24, 25 have a third value for the time period, 120 ms
which lies in between the values 60 and 240 ms. The scan rates for
each type of field device are known beforehand. The scan rates for
each control loop are specified in the control logic, 60 ms, 120 ms
etc., based on the known update rates for each field device in the
control loop. That information on scan rates is available and
specified, for example, in the control system configuration.
[0055] FIG. 3 shows three superframes 30, 32, 34, described here
beginning at the top with the 60 ms scans or updates. As stated
above, in WirelessHART in order to support multiple superframes for
the transfer of process measurements at different rates, the size
of superframes should follow a harmonic chain. This means that all
periods should divide into each other, in particular, scan rates
should be configured as integer multiples of the fastest update
time that will be supported by network devices. Thus the 240 ms
periods include multiples of 60 ms and 120 ms.
[0056] Thus in the first wireless communication schedule,
superframe 30, there are four cycles each 60 ms apart. Note that
there is a dependency between field devices in this control loop
because sensor S4 shall send its signal to the controller before
the controller carries out its control task C3 in a control cycle
and then communicates its signal to actuator A4. One sensor input
S4, cycles four times in the superframe as does one output to
actuator A4. Similarly time slots may be allocated in the second
superframe 32 to two control loops 24, 25 cycling at 120 ms. Three
separate sensor inputs S5, S6, S7 cycle twice in the superframe; as
do two actuator outputs A5, A6. (Note that in the example shown
control loop 23 has two sensor inputs S6, S7.) The third superframe
is for the slowest cycle in the group of control loops of interest,
that is to say, the slowest period 240 ms in the wireless network
of interest. This superframe includes one 240 ms cycle only for
each of two control loops 21, 22. The superframe has time slots for
one single input from each of sensors 51, S2, S3 and time slots for
one single output from each of A1, A2, A3. Note that this
superframe is arranged to comply with the dependency of control
loop 21 on loop 22; ie that control loop 22 completes before the
control task C1 of loop 21 may be carried out by a controller,
leading to the single output to actuator A1 in this superframe.
[0057] FIG. 3 shows a controller task schedule 36 arranged in a
superframe. Wireless communication schedules 30, 32, 34 may be
executed jointly with the controller task schedule 36.
[0058] FIG. 4 shows a flowchart for method of automatically
generating a wireless communication schedule based on dependency
between two field devices in a control loop, which may be
represented as a dependency chart or dependency graph. The figure
shows:
[0059] 40 Importing dependency information for the field devices in
a selected one or more control loops, from a CAD file and/or a
P&I diagram, and/or process control logic configured in a
control system or control system tool, process logic arranged in
one or more operator process graphics, or in another electronic
schema;
[0060] 46 Allocating time slots for field devices in the one or
more control loops in at least one superframe dependent at least on
the dependency information between two field devices (in this
example the field devices are sensors or actuators) and optionally
allocating time slots in a control task schedule 36 for a control
task carried out in a controller dependent at least on the
dependency information between two or more field devices and/or
between two or more control loops.
[0061] FIG. 6 shows a flowchart for an alternative method. The
flowchart shows the steps of
[0062] 40 Importing dependency information for the field devices in
a selected one or more control loops, from a CAD file and/or a
P&I diagram, and or process control logic configured in a
control system or control system tool, process logic expressed in
one or more operator process graphics, or in another electronic
schema;
[0063] 42 Optionally automatically or semi-manually converting the
information on dependency between field devices into a dependency
relationship such as a dependency graph;
[0064] 44 Optionally identifying a longest time period that
includes an integer multiple of all shorter time periods (scan
rates or updates rates)
[0065] 46 Allocating time slots for field devices in the one or
more control loops in at least one superframe (30-34) dependent at
least on the dependency information between two field devices
(sensors or actuators) and optionally allocating time slots in a
control task schedule (36) for a controller dependent at least on
the dependency information between two field device and/or between
two control loops.
[0066] FIG. 7 shows a flowchart for a method according to a
preferred embodiment. The flowchart shows the steps of
[0067] 50 Importing information for dependency between one first
control loop and another second control loop, from a CAD file
and/or a P&I diagram, and/or process control logic configured
in a control system or control system tool, and/or a process
graphic, or in another electronic schema;
[0068] 56 Allocating time slots for field devices in at least one
control loop in at least one superframe and/or control task
schedule for a control task dependent at least on the dependency
information and optionally allocating time slots in a control task
schedule (36, 37) for a controller dependent at least on the
dependency information between two field devices and/or between two
control loops.
[0069] In addition to the 10 ms allocated time slots described
above, standards such as the WirelessHART standard also permit that
a time slot may be shared by two or more devices or nodes. Within
the above standards there are methods to cope with data collisions
in a time slot, so that shared time slots may be allocated. For
example when data traffic is expected to be low a time slot may be
shared, leaving collision avoidance mechanisms such as random time
back-off to take care of conflicts occurring less often, for
example during a configuration task or an optimising procedure at
the installation or in the process. To take advantage of low
traffic for certain communications in a network one or more time
slots known to have low usage may be configured as shared time
slots. This has the advantage of increasing the efficiency of
bandwidth use. A random back-off time may be used, or an adaptive
method that adapts to measured or estimated collisions occurring. A
fixed back off time or a series of predetermined Wait times or
Guarantee Time Slots (GTS) may also be used.
[0070] The dependency information is preferably imported into an
application or function of the DCS. One optional source for
dependency information is from a controller configuration in a
control system, or such a configuration created in a DCS. A
procedure to examine, edit or create a controller configuration may
be run from any terminal connected to the DCS, such as an operator
terminal, engineering terminal or workstation, a portable or
handheld device or similar. Dependency information, whether in the
form of a controller configuration or an imported P&I Diagram,
CAD file or other format, is converted preferably automatically by
the application or function of the DCS into one or more time slot
allocation schemes or superframes. A copy of the one or more
superframes thus created is preferably stored in the wireless
gateway 60 of the wireless network in question where it is used by
the Network Manager application or function to control wireless
communication between the nodes 51-57, 60, 62, 64, 65 in the
wireless network (see FIG. 5 below).
[0071] FIG. 5 (Prior Art) shows a typical diagram for a
wirelessHART network. The figure shows a number of wirelessly
enabled field devices or nodes 51-57, a router 62, one or more
network access points 64, a wireless gateway 60 and an industrial
automation or Plant Automation network 50. The wireless gateway 60
is the gateway handling all wireless communication to/and or from
the field devices to the process control system. A handheld device
65 is indicated, which is used for configuring, diagnosing and
similar tasks in the network or in respect of a particular
node.
[0072] A Network Manager application or device (not shown) is
arranged either in the Gateway 60 or else directly connected by a
secure connection to the Gateway. In this description securely
connected means that the communication path between the wireless
gateway and the network manager is a secure communication channel,
typically a hard-wired connection. In a wireless network such as a
WirelessHART network, the Network Manager unit or application is
responsible for configuration of the network, scheduling
communication between network devices, management of the routing
tables and monitoring the health of the WirelessHART network.
[0073] FIG. 9 shows a process and instrumentation (P & I)
diagram for a chemical reactor vessel 90. The figure shows the
reactor vessel with a stirrer 92 and a cooling jacket 94. The
reactor vessel 90 is further arranged with an inlet for coolant 96
which is controlled by a flow control valve FV103. The temperature
inside the reactor vessel is monitored by a sensor, temperature
transmitter TT103 and the cooling jacket 94 is similarly arranged
with a temperature sensor and monitored as temperature transmitter
TT102. An outlet 98 is arranged to allow coolant to exit the
cooling jacket. Furthermore the coolant temperature, measured by
temperature transmitter TT101, is separately controlled using a
heating valve FV101 and cooling valve FV102.
[0074] The inflow of component A to the reactor vessel is
controlled by flow control valve FV104. In this exemplary
embodiment, components A and B are to be introduced according to a
ratio. A ratio station RS multiplies the flow of A, given by the
flow transmitter FT104, with the desired ratio and then controls
the flow of B (FT105) into the reactor via flow controller FC105
via flow control valve FV104.
[0075] This example illustrates 4 control loops 111-114, each
indicated by a box shown with dashed lines, arranged around a
stirred chemical reactor 94, and how communication and control
tasks could be scheduled if both sensors and actuators are
wirelessly connected to the controller. A P&I diagram for this
example is shown in FIG. 9 and a list of all instruments and valves
is given in Table 1 below.
TABLE-US-00001 TABLE 1 Instrumentation and valve list. TT101
Coolant temperature TT102 Reactor temperature TT103 Jacket
temperature FT104 Feed flow component A FT105 Feed flow component B
FV101 Heating valve FV102 Cooling valve FV103 Coolant flow valve
FV104 Feed valve component A FV105 Feed valve component B
[0076] As can be seen from the P&I diagram there are separate
control loops 111 and 112 of the coolant temperature and the
coolant flow. The temperature is controlled by a split range
controller TC101 which opens the heating valve FV101 if coolant
temperature is below setpoint and cooling valve FV102 when
temperature is above setpoint.
[0077] The coolant flow is then used to control the temperature of
the reactor using a cascade control structure. Here the reactor
temperature is used in a master controller TC102 which provides
setpoint to a slave controller TC103 controlling the jacket
temperature, which in turn commands the valve opening FV103, in
control loop 112. Although this structurally is depicted as two
controllers, the execution of a cascade is usually performed as one
calculation needing both process measurements as input. Hence in
the scheduling these two controllers are jointly denoted as
TC102-3.
[0078] Finally, it is here assumed that there are two main
components fed into the reactor; components A and B, and that their
flows need to be in a certain ratio to each other. Therefore there
is only one independent flow loop 113 controlled by FC104, which
setpoint indirectly determines the total feed flow. The flow of
component B is then controlled by a ratio control structure.
Similar to the cascade controller, execution of the ratio station
RS and the flow controller FC105 are typically done as one joint
control task (in the schedule denoted only as FC105).
[0079] Controller Configuration
[0080] Exactly how this set of 4 controllers (or 5 controllers if
you count master and slave of the cascade separately) is configured
in the control system may vary of course depending on the
particular system used. However, if the configuration is done
graphically using a graphic user interface and methods such as drag
and drop it should typically look something like FIG. 10.
[0081] Scheduling
[0082] For the sake of this small example we assume that all
control loops run using the same cycle time 100 ms. This is
probably unnecessarily fast, but will serve the pedagogic purpose
that it only allows a superframe of 10 slots for a WirelessHART
schedule (since slots are 10 ms long), which incidentally is
exactly the number we need for a single-hop schedule.
[0083] Assuming that a control task can be completed within one
time slot, an example of optimized communication and control
superframes, yielding a minimum of latency between sensing and
actuating, is shown in FIG. 12.
[0084] Preferably the dependency graph is generated automatically
based on either a CAD process and instrumentation (P&I) diagram
or based on a controller configuration in a control system or a
Distributed Control System (DCS). Alternatively the time slots may
be calculated or otherwise determined directly from the dependency
information without an intermediate step of constructing a
dependency chart or dependency diagram as such.
[0085] FIG. 10 represents a simplified screen display for an
application 100 for configuring a process control system or DCS.
The figure shows five control task objects which are labelled
TC101, TC102, TC103, FC104, FC105. These objects have been arranged
to configure the process control for the reactor shown in FIG. 9.
Thus the control object 105 representing flow controller FC105 has
inputs 103, which for this control object 105, are a flow
transmitter FT105 (flow of component B), a second flow transmitter
FT104 (flow of component A) and a Ratio station with a
predetermined setting Ratio representing a desired ratio for B:A.
Control task object 105 has in this case one output 107 to an
actuator which is flow control valve FC105, controlling the inflow
of B into the reactor. The other four control task objects are
similarly configured with inputs and outputs to represent the
process flow and control diagrammed in FIG. 9.
[0086] FIG. 11 shows a dependency chart for the process shown in
the FIG. 9. The dependency chart shows 4 control loops 111-114. The
dependency chart shows, for example, the control loop 112
controlling flow control valve FV103 controlling coolant flow into
the reactor as follows. Inputs of temperature from sensors TT102
(reactor temp) and TT103 (cooling jacket temp) are input to a
controller. In this case the controller is indicated as TC102-3
because these master/slave control tasks are typically executed
together as one joint CPU task. The controller output goes to the
actuator flow control valve FV103 to adjust the coolant flow into
the cooling jacket. The dependency chart may be produced
automatically by converting the P&I diagram of FIG. 9, or the
process controller configuration in a control system or a
Distributed Control System (DCS) as illustrated in FIG. 10.
Alternatively or as well a dependency relationship between objects
in a control loop may be extracted from process graphics, in
particular from operator process graphics. The dependency chart may
be used to automatically generate one or more superframes or
communication time slot allocations, in the same way as the
communication time slot allocations 30, 32, 34 and the control time
slot allocation 36 of FIG. 3.
[0087] Alternatively the time slot allocations may be automatically
produced directly from a P&I diagram such as FIG. 9, or a
control application configuration schema such as FIG. 10, without
actually producing a dependency chart output of the type shown in
FIG. 11.
[0088] FIG. 12 shows a frame with communication time slot
allocation for the control loops 111-114 of the process of FIG. 9.
Each control loop includes wirelessly enabled field devices, for
example sensors TT101-TT103 and FT104-105 of FIG. 9, and actuators
representing actuator valves FV101-FV105. It should be noted that
one or more field devices of a control loop, for example one or
more actuators, may have a wired connection to a controller and not
a wireless connection. In any case, each control loop includes a
control task representing controllers TC101-TC103, FC105-FC105,
which are typically executed in one or more Programmable Logic
Controllers (PLC) or other industrial controller. More than one
control task may be executed in a single controller. The
communication superframe 35 includes sensor cycles TT101-103,
FT104-105 and actuator (FV101-FV105) cycles each of 100 msec. The
controller time slot allocation or controller superframe 37 shows
one time slot allocation per superframe for each of the control
tasks TC101, TC102-3, FC104-105, assuming that they will all be
executed in the same controller CPU.
[0089] In an advantageous application of the invention, time
synchronisation in the wirelessly enabled control loops is
synchronised with a clock signal from the clock in an industrial
controller based on a signal from the wireless gateway to the
controller. This ensures that the control cycle of one or more
controllers is synchronised with the one or more TDMA-based
superframes with communication time slots for field devices when
sending and/or receiving is accurate and maintains quality of
wireless service.
[0090] The invention has been described in relation to wireless
networks compatible with the WirelessHART standards, but may with
suitable adaptation be practised with other TDMA based networks
compatible with an industrial standard such as that know as
"Fieldbus", and is specifically referred to as SP50, which is as
acronym for Standards and Practice Subcommittee 50, or to ISA 100
standard.
[0091] The microprocessor (or processors) of a wireless controller
carrying out a control task such as Flow control 105, master
control TC102 or Slave control TC103 comprises a central processing
unit CPU performing the steps of the method according to an aspect
of the invention. For example carrying out one or more control
tasks C1-C5. These steps are performed with the aid of one or more
computer programs, which are stored at least in part in memory
accessible by the processor. It is to be understood that the
computer programs may also be run on one or more general purpose
industrial microprocessors or computers instead of a specially
adapted computer.
[0092] The computer program comprises computer program code
elements or software code portions that make the computer perform
the method using equations, algorithms, data and calculations
previously described, for example in relation to FIGS. 4, 6-8. A
part of the program may be stored in a processor as above, but also
in a ROM, RAM, PROM or EPROM chip or similar memory storage device.
The program in part or in whole may also be stored on, or in, other
suitable computer readable medium such as a magnetic disk, CD-ROM
or DVD disk, hard disk, magneto-optical memory storage means, in
volatile memory, in flash memory, as firmware, or stored on a data
server. Removable memory media such as a USB memory stick, flash
drive, or similar.
[0093] It should be noted that while the above describes
exemplifying embodiments of the invention, there are several
variations and modifications which may be made to the disclosed
solution without departing from the scope of the present invention
as defined in the appended claims.
* * * * *